This invention relates generally to semiconductor packaging. More specifically, the invention relates to the design and manufacturing process of chip carrier bond pads which increase the quality and reliability of the interconnections between semiconductor chips and circuit boards.
Ball grid array (BGA), chip-scale package (CSP), and solder-bumped flip chip technologies are all commonly used packaging technologies where solder is the electrical and mechanical "glue." Solder joint reliability is one of the most critical issues in the development of these technologies.
A semiconductor package can have conductive beads forming these solder joints. These electrically conductive beads can be in the form of solder balls. For example, a ball grid array (BGA) process can be used to apply solder balls to the underside of a semiconductor package for subsequent bonding of the package to a circuit board for use. A BGA package is placed so that each solder bump of the BGA package contacts the corresponding pads on the circuit board. The solder is heated to induce reflow and electrical connections. These connections serve to secure the semiconductor package to the printed circuit board and to electrically interconnect at least one semiconductor chip carried in the semiconductor package with the metallizations on the circuit board. Such semiconductor packages are used extensively in the electronics industry because of their compact size, ease of mounting, and versatility.
Ball grid array packages are available in a variety of different configurations and substrate materials. These include Ceramic Ball Grid Arrays (CBGA), Ceramic Column Grid Arrays (CCGA), Tape Ball Grid Arrays (TBGA), and Plastic Ball Grid Arrays (PBGA).
The solder pads for a BGA package, chip-scale package (CSP) or for flip chips, which are unpackaged semiconductor carriers whose bonding pads bear solder bumps which are substantially truncated-sphere-shaped solidified solder, are often currently manufactured in a process that involves depositing nickel and subsequently gold on the surface of copper exposed through a dielectric layer. It is well known in the art that electroless and electrolytic plating processes can be used for depositing nickel, and electroless, typically immersion, and electrolytic processes can be used for depositing gold. The copper at the bond pad interface can be deposited or electroplated. Solder balls for semiconductor package interconnects are often composed of a eutectic tin-lead alloy. These tin-lead balls form an intermetallic compound with nickel when soldered onto the bond pad.
Electroless nickel metallization can occur after the cooper has been deposited, imaged and etched and can occur before or after the application of soldermask. Nickel covers all exposed copper surfaces and acts as a barrier between the copper and the subsequently deposited gold. This nickel layer is required to prevent the formation of a compound between gold and copper that can not be soldered. The nickel also adds strength to the bond pad. This process is well known in the art, as described in U.S. Pat. No. 5,891,756, for example, the entire specification of which is incorporated herein by reference for all purposes.
Immersion gold plating occurs after the electroless nickel plating and after etching. Gold covers all exposed nickel surfaces and acts as a bond metal. The immersion (electroless) gold plating process has distinct advantages over its electrolytic counterpart. For example, the immersion gold process has the capability of depositing a much thinner layer of gold than the electrolytic process.
In the immersion gold process, the gold can also fully cover the nickel because electroless nickel and immersion gold can be performed after etching, as no electrical connections for electrolytic nickel or gold plating are required. However, it is well known in the art that during electroless deposition of gold, corrosion of the nickel barrier layer often occurs due to the electrochemical nature of the process. The corrosion of the nickel layer results in nonuniform gold deposition. Excess gold may not be completely soluble during reflow and attachment of the solder balls causing a weak ball joint. Excess gold also prevents the solder balls from forming an intermetallic compound with the nickel.
FIG. 1 illustrates how the corrosion of the nickel may occur when convention device designs and methods are used. FIG. 1 is a cross-sectional view of a ball joint on a bond pad. A conductive pad 130, commonly made of copper or aluminum, is formed on a substrate 150. This conductive pad 130 can be treated to allow better nickel adhesion when electroless nickel plating occurs. Micro-etching can clean the surface of the conductive pad as well as create a toothed structure on the surface of the conductive pad. A catalyst can also be applied to further increase the ability of nickel to adhere to the pad.
Nickel 120 can then be electrolessly plated onto the pad 130. A common deposition rate for the nickel bath is approximately 10 microinches per minute. A nickel 120 layer thickness of 50-250 microinches is often specified to prevent a subsequently deposited gold layer from diffusing into the copper. Nickel provides good hardness, uniformity, and corrosion resistance.
Unfortunately, nickel 120 itself may corrode when the immersion gold process deposits gold 110 onto the surface of nickel. A layer of gold 4-7 microinches thick is common and is a possible result of the immersion gold process. However, corrosion of nickel 120 during immersion gold results in uneven deposition of gold 110. Too little gold 110 is deposited where the nickel 120 is corroded and too much is deposited where the nickel 160 is not corroded. This unevenly deposited layer of gold may not be completely soluble during reflow (i.e., where gold deposition is too thick), and a eutectic solder ball 100 often comprised of tin-lead cannot form a complete interconnection with the nickel 120. This incomplete connection, results in dewetting, poor adhesion, and low reliability.
In an ideal solder joint, the surface is wetted when the solder is heated and the result is a continuous, unbroken film. With excess gold deposition, dewetting may result where the solder balls appear to have wetted the surface of the nickel but leave an irregular, nonuniform joint.
Accordingly, what is needed are methods and apparatuses for improving the reliability of semiconductor packages by providing more reliable ball joints to minimize dewetting during soldering and to increase the adhesiveness of the intermetallic bond formed between the solder ball and the barrier metal.